A Better Wall with Exterior Foam
Why a longtime SIP builder turned to rigid polyiso for insulation.
Synopsis: Builder Jonathan Orpin was a diehard fan of SIPs and used them in countless projects. Over the years, though, he started to realize that manipulating SIPs for sidewalls required a lot of work when cutting openings and routing mechanicals. To get around this problem, Orpin developed what he calls the matrix wall, which consists of conventionally framed 2×6 walls with either wet-sprayed or netted dense-pack cellulose in the cavities and polyisocyanurate foam in the exterior. Housewrap and a rainscreen of 1x strapping serve as a weather-resistant barrier. Orpin says the assembly yields an R-30 wall with negligible thermal bridging and a managed dew point. This article includes sidebars on controlling moisture in walls and a better way to install windows.
I’ve been a home designer, builder, and timber framer for more than 25 years, and I recently calculated that my projects have consumed almost 2 million sq. ft. of SIPs…
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